FIG. 7(a) shows a conventional oscillation circuit using ring oscillators which oscillates in such a state that two ring oscillators 102 and 103 each composed of an odd number of CMOS inverters 101a-101c are cross-coupled by using inverters 104 and 105. In this case, by virtue of the cross-coupling using CMOS inverters, an oscillation circuit having good duty controllability can be attained.
FIG. 8 shows a ring oscillator constituted of a plurality of current inverters 106. Because in this ring oscillator a signal is transmitted in a current mode, it is more suitable for increasing the operation speed than ring oscillators of a type using CMOS inverters in which a signal is transmitted in a voltage mode (voltage is an integration of current). The current inverter has a feature that it produces less switching noise than the CMOS inverter because a transistor on the power supply side or the ground side serves as a constant current source.